Liquid developers for developing electrostatic images



United States Patent 3,244,633 LIQUID DEVELOPERS FOR DEVELOPING ELECTROSTATKC IMAGES Wilbur Yellin, Chenango Forks, Donald E. Rutter, Jr.,

Vestal, and John W. Weigl, Bingharnton, N.Y., assignors to General Aniline 8: Film Corporation, NewYork, N .Y., a corporation of Delaware I No Drawing. Filed Oct. 24, 1961, Ser. No. 147,202.

6 Claims. (Cl. 252-621) This is a continuation-in-part of our application Serial No. 68,604, filed November 14, 1960, now abandoned.

This invention relates to electrophotography and to the production of electrophotographic images. More particularly, the invention is concerned with liquid developers having predictable and reliable electroscopic properties and the use of'such developers for producing visible images having improved qualities, such as higher degree of resolution, permanence and the like.

It is known to provide visible images by electrophotographic means, a technique generally referred to as electrophotography. In a commonly-used form of this system of photographic reproduction, a photoconductive layer coated on a relatively conducting support, is provided with a surface electrostatic charge. On exposure to a radiation pattern, or any other source of modulated energy, the electric charges are dissipated or otherwise removed in proportion to the intensity of the exposing radiation. As a consequence, there is retained on the surface of the photoconductive material a charge pattern, the charge distribution of which varies inversely With the original exposure. Areas of the photoconductive layer receiving an intense exposure retain only a slight residual charge density, whereas, those areas of the plate receiving low exposure retain a relatively high charge density.

The electrostatic charge pattern, commonly referred to as an electrostatic latent image, can be transformed into a visible image by a toning or development technique familiar to the electrophotographic art. For instance, a general procedure for developing electrostatic images consists of applying to the exposed electrophotographic layer a finely divided developer or toner powder which deposits on the latent image in proportion to the density of the charge pattern with concomitant production of a visible image.

In developing electrostatic images, recourse may be had to a wide variety of development techniques depending on the type of image desired. Particles of the toner can be charged opposite in polarity to that of the latent electrostatic image, in which case a visible image is obtained by attraction development. Alternately, toner particles can be utilized which are identical in polarity to that of the latent image and are, therefore, repelled from it and consequently deposit or adhere preferentially upon uncharged surrounding areas. This latter procedure is known as repulsion development, and yields a visible image opposite in character to that obtained by attraction development. Control electrodes of the type described, e.g., by Butement- (Gcrman'application 1,093,666, 1960, to Commonwealth of Australia) are useful for controlling field direction andintensity.

Developers for use in electrophotog-raphy may consist of finely divided powders which, on application in the form of a dust, are retained by the residual charge pattern on the photoconductivelayer. Such materials'must he possessed of reliable and proper electroscopic properties if sharply differentiated images are" to be obtained.

Developed electrostatic images must also be fixed or otherwise rendered permanent to prevent their being smudged or dislodged from the support in handling operations. One often practiced method is the application of "ice heat, which melts the powder particles constituting the image with-resultant bonding of the fused resin to'the substrate. Another method is to coat the toner particles constituting the image with a tacky resinous substance which bonds the toner particles both to each other and to the image substrate. Other less practiced methods are disclosed inthe literature.

Because they must possess a rather restricted and narrowly defined family of properties, including bothelectrical and physical characteristics, the design and fabrication :of practical electrophotographic toner. compositions suitable for commercial uses is indeed a difiicu-lt problem and still confronts this area of the electrophotographic field. i

A substantial improvement in electrostatic development is described in the US. Patent 2,907,674 to Metcalfe and Wright. This patent-discloses electrostatic developers or toners comprising a suitable electrostatic powder of extremely fine particle size dispersed in a liquid of high electrical resistivity. In carrying out the development process in accordance with the patent, anelectrostatic image is wetted by the aforementioned liquid developer whereby the suspended particles are caused to adhere-to the electrostatic image by migration through the carrier liquid with subsequent development of a visible image. The carrier layer may also contain a separate fixing agent for the purpose of rendering the pigment fast to the image'- bearing surface when the carrier liquid is evaporated or otherwise removed. The salient feature of the Metcalfe et al. technique is the utilization of a resinous control agent, the function or purpose of. which is to adhere to or coat, or otherwise intimately associate with the pig;- ment particles in order to produce a developer having the correct electrical polarity with respect to: the electrostatic image. In other words, the electrical properties of pigment particles, which are, in themselves, notoriously erratic, are governed or controlled by the coating of the resinous control agent. Consequently, the electrical characteristics of. such toner compositions are more reliable than when the pigments per se are suspended or dispersed in a high'resistivity carrier liquid, asfeatured in some of the earlier liquid toner developers disclosed, e.g., by Metcalfe and Wright, J. Sci. Instruments 33, 194 (1956).

Although the liquid development of electrostatic images as proposed in the Metcalfe et al, patent represents an improvement overv the previously used developers, the difiiculties in selecting a proper resinous control agent has heretofore prevented the full utilization of this development system. We have, for instance, prepared liquid toners containing a negative'control agent in'ac'cordance with the aforementioned Metcalfe et al. procedure and whereas these toners operated satisfactorily if used shortly after their formulation, the effectiveness rapidly diminished and after a few hours from the time of preparation such toners were unsuitable for practical work. A likely explanation for this behavior is the gradual dissolution of the sheathor capsule of the resinous control agent by the solvent action of the dielectric carrier liquidwith resultant baring or exposure of the bare pigment surface. This would account for the change in electrical characteristics of the Metcalfe et al. developer compositions after standing for a short time.

We have now discovered liquid developer compositions for developing electrostatic images which have excellent electrical and physical qualities and, furthermore, are especially valuable in that their electrical. propertiesv do not undergo alteration on prolonged standing. Said liquid developers, a method for producing them and the development of electrostatic images thereby constitute the primary objects and purposes of this invention.

We have ascertained that reliable liquid toners capable of providing electrophotographic images of high definition can be produced without resorting to the afore-described control agents, if the proper electrical or electrostatic relationship is established between the pigment particles on the one hand and the fixing agent on the other. By eliminating the capsule or sheath of control resin surrounding the pigment particle, our liquid toners are not subject to the instability which plagues the formulations of the prior art. Specifically, it minimizes the alteration in which R is hydrogen or alkyl as above.

(4) An epoxy resin of the type manufactured by the Shell Chemical Corporation under the trademark Epon 1001, said resin being characterized by the following formula:

CH3 in electrical properties on prolonged standing because of the leaching or dissolving away of the control resin sheath by the solvent action of the carrier liquid.

Our invention is predicated on the discovery that tonor compositions can be formulated by selecting fixing agents and pigments which have strong zeta potentials of the same polarity. By thus ascertaining and establishing the proper electrical relationship between the pigment particles and the fixing agent, our toner compositions are not subject to alteration of electrical characteristics. This is due to the fact that we do not employ encapsulating control resins which, as previously pointed out, may undergo dissolution by the carrier liquid with concomitant erratic electrical behavior of the toner composition. By utilizing a more basic approach, i.e., proper electrical relationship between pigment and fixing agents, our procedure provides liquid toners which are stable over practical time limits and are thus applicable for commercial use.

A further advantage of the instant invention is the utilization of a single component serving both as the fixing agent and as the electrical regulator for the pigment particles. In this way, we have eliminated the need for preparing formulations containing a separate fixing agent and a separate control agent.-

In preparing our liquid developers, we disperse pigment or toner particles in a carrier liquid of high electrical resistivity in the presence of a resin fixing agent, preferably by milling the fixing resins in contact with the pigment although resort may be had to other blending methods. Resinous fixing agents which are illustrative of the type suitablefin practicing the invention are exemplified in the following lists:

. A. Positively charged fixing agents for the direct or attraction development of negative electrostatic images include:

(1) Styrene and butadiene copolymers (such as are sold by Goodyear as Pliolite SS-B) having the recurring unit (Z) Acrylate and methacrylate copolymer resins (such as are sold by Rolmi & Haas Co., as Acryloid A-lOl) having the recurring unit in which R and R may be hydrogen or lower alkyl, i.e., methyl, ethyl, propyl, butyl or the like. I (3) Aldehyde dominated alkyd resins (such as are sold CH; An unusual feature of this resin is the absence of an additional curing agent normally required for epoxy-type resins, when used as fixing agents for electrophotographic processes.

B. Negatively charged fixing agents for reversal or repulsion development of negative electrostatic images include the following types:

(l) Styrene acrylonitrile modified alkyd resins (such as are sold by American Cyanamid Company as Cycopol 320- containing the recurring unit:

(2) Vinyl ether resins (such as Butvar which can be procured from the Shawinigan Resins Corp.) having the recurring unit:

wherein R is alkyl as above or an aromatic group such as phenyl, toluyl or the like.

(3) Poly(vinyl alcohol-paranitrophenyl ether) resin (available from the General Aniline and Film Corp.) having a recurring unit represented by the following formula:

In the above formulae, n is a whole number equal to at least 5.

The aforedescribed types of polymers, copolymers and resinous substances are well known in the electrophotographic arts and can be readily purchased on the market under their well known trade names and trademarks. It will be understood that toners formulated with positively charged fixing agents will be selectively repelled from positively charged areas and that those based on negatively charged fixing resins will be selectively attracted towards positively charged areas.

Some predominantly positive pigments suitable for practicing the invention are the following:

GENERAL DYESTUFFS PIGMENTS Permanent Carmine FR Toner 35-500l (CI. Pigment Red 5) Heliogen Blue DS Toner 56- 5001 (Cl Pigment Blue 15) Heliogen Blue LE Toner 56- 5001 (CI. Pigment Blue 15) Pigment Green B Toner 65- 4001 (CI. Pigment Green 8) Pyrazolone Red B Toner 31- (CI. Pigment Red 38) Permanent Red F4R 35400l (CI. Pigment Red 8) Permagen Red D Toner 3 1- 5091 Permanent Violet Toner 49- 6091 (CI. Pigment Violet 3) (CI. Pigment Red 3) E. I. DU PONT DE NEMOURS & CO., INC.

Toluidine Red (CI. Pigment Red 3) Watchung Red B (CI. Pigment Red 48) Monastral Blue B (CI. Pigment Blue Benzidine Yellow (CI. Pigment Yellow 12) NATIONAL ANILINE AND DYE PIGMENTS Iosol Black (CI. Solvent Black 13) Spirit Nigrosine SSB (C.I. Solvent Black 5) MISCELLANEOUS PIGMENTS Leafed aluminum powder Zinc oxide Lead oxide Titania Lycopodium powder Predominantly negative pigments are more rare but include:

GENEKAL DYESTUFFS PIGMENTS Helogen Green Toner 66- 3001 (CI. Pigment Green 7) Talcum powder Sulfur Pigments which can acquire either charge are more common and include:

GENERAL DYESTUFFS PIGMENTS Heliogen Blue BGN Toner 56- 6091 (CI. Pigment Blue 15) Magnesium oxide Iron oxide Cobalt oxide Manganese oxide Lead chromate Carbon black Pigmented materials which we have found particularly useful in connection with the previously enumerated fixing agents are various hydrophobic black inks for use in offset and other printing processes. Examples of these substances include Offset Barta and Griffin Rocket Speed Black, Colitho Miracle Black, Colitho Black 1A, Colitho Reflex Blue, Colitho Dark Red, Colitho Bright Green CO 28, Sinclair and Valentine Offset Duratree Black, Sinclair and Valentine Offset Duratrace Black, IPI Special Tracing Paper Black. A good general discussion of the formulation of printers inks may be found, e.g., in the Pressmens Ink Handbook by H, I. Wolfe (Dorland Books, New York, 1952).

When utilizing printing inks in producing our electrophotographic image, we have ascertained that the density and degree of polarity control varied somewhat with each individual printing ink and resin system. For the most cases, ten to tweny-five percent of resin (based on ink weight) successfully fixed the ink with no decrease in polarity of the ink particles. It will be understood, of course, that other positively controlled inls may be easily substituted for any of the above inks and fixed with such resins as are indicated herein. In addition, pigments whose zeta potential and polarity control is not well regulated or which contain particles of both polarities and thus tend to tone images erratically, can be made more reliable in attraction to a charged surface by incorporating as a fixer additive in the stock solution a resin whose zeta potential and polarity control is of a desired character. Thus, carbon black pigments which are notorious for erratic behavior due to the presence of particles of both polarities can be made to exhibit reliable positive polarity control in the presence of a butadiene-styrene copolymer resin of high positive zeta potential, and negative polarity control in the presence of a vinyl chloride cop-olymer resin of negative zeta potential.

The weight ratio of dry pigment or of ink pigment solids to resin may be conveniently varied from 10:1 to 1:10

6 with a corresponding variation in adhesion and density of the developed image. Excess pigment leads to a dense image with poor adhesion and excess resin gives good adhesion with low image density. Effective polarity control and adhesion of the toner particles is observed in this range.

In addition to the milling and solvating liquids, spelled out in the examples, we have utilized toluene, xylene, mesitylene, benzene, carbon tetrachloride, methylene chloride, chloroform, methyl chloroform, trichloroethylene, perchlorethylene, Freon 112, and other aromatic and/or halogenated hydrocarbons or mixtures thereof having suflicient resin solvency. We have found that it is also possible to use limited quantities of polar solvents when they are required to dissolve the resin. For example, we have added up to 30% of mcthylethyl ketone or 50% of butanol (percentages based on volume of solvating solvent, not diluted toner mixture).

Milling and dispersing of pigment in the resin can be accomplished in pebble mills, triple roll mills, sand mills, homogenizers, sonic blendors and other means known in the art for appropriate periods of time varying up to 100 hours. Non-solvating, non-conductive dispersing liquids known in the literature include Freon 11, Freon 113 (1,2, 2-trichloro-l,1,2-trifluoroethane) and other fiuorinated hydrocarbons, petroleum ether, ligroin, pentane, hexane, heptane, cycl-opentane, cyclohexane, kerosene, other saturated hydrocarbons, and mixtures of the above whose electrical resistivity is of the order of 10 ohm centimeter or greater. The percentage of dissolving solvent in the final toner bath is conveniently higher than the minimum required to insure compatibility of the resin in the dispersing liquid, but low enough to insure sufficient electrical resistivity of the order of 10= ohm centimeter or greater in the final toner bath.

The following examples will serve to illustrate the invention, but it is to be understood that such examples are inserted for the purpose only of facilitating the practice of the invention and not by way of any limitation.

Example I 2.0 grams of a 15% butadiene styrene copolymer resin (purchased from the Goodyear Company under the trademark Pliolite 85-13) was dissolved in grams of toluene to form a fixer solution. 15 grams of a black lithographic offset ink (procured from the B-arta and Gritfin Company, Worcester, Mass, as Offset Rocket Speed Black) was dispersed in 100 grams of toluene to form a dense black stock solution. The dispersing was effected by means of a Waring Blendor of the type commonly employed in laboratories for this purpose. 10 ml. of the fixer solution was combined with 10 ml. of the stock solution and the resulting mixture diluted with 280 ml. of Du Pont Freon 113 to form a positive rapid fixing toner bath. Negative charge image patterns, formed on an electrophotographic member comprising a humidified paper base having coated thereon particles of zinc oxide suspended in electrically insulating medium were fully developed to a dense lustrous black visual image upon immersion in the toner black for approximately 5 seconds. After removal from the developing bath the so-obtained image was resistant to smearing or smudging after a few seconds. The efficiency of the above-described toner formulation was tested in the following manner:

A thin white sheet of paper was placed in contact with a grounded metallic plate having set therein 11 coplanar insulated metallic plaques individually connected to potential sources of +60, +120, +235, +420, +600, +70, +130, +200, +320, +470, +600 volts respectively. A sample of the liquid toner prepared in accordance with the instructions of Example I was poured onto the insulating paper sheet and spread over the surface with a grounded metal roller. In effect, the roller and the plaque form 11 differentially charged electrode pairs which serve to preferentially attract or repel the controlled toner particle.

Examination of the paper sheet after evaporation of the Freon 113 carrier fluid showed a pattern of black spots corresponding in size, shape and location to the negativelycharged plaques. In addition, the regions on the paper directly over positively charged plaques were entirely free of black toner even though the surrounding paper areas did absorb some black coloration.

Example 11 The procedure of Example I was followed except the Ofiset Rocket Speed Black ink was replaced by Colitho Miracle Black ink manufactured by the Colitho Division of Columbia Ribbon and Carbon Manufacturing Co., Inc., Glen Cove, New York. Results paralleled those of the first example.

Example III In this example, the lithographic ink was Colitho Reflex Blue. For the rest, the procedure and materials of Example I were employed. A lustrous blue visual image was produced.

Example IV Colitho Dark Red was substituted for the Offset Rocket Speed Black of Example I. The procedure and results were approximately in consonance with the earlier examples.

Example V The lithographic ink was an Offset Duratree Black procured from the Sinclair and Valentine Company. This formulation gave results nearly identical to those obtained in Example I.

Example VI In this example, the lithographic ink was an Offset Duratrace Black obtained from Sinclair and Valentine Company and the procedure and quality of image paralleled those of the earlier examples.

Example VII In this example, the lithographic ink was a Special Tracing Paper Black manufactured by the Printing Inks, Division of Interchemical Corp. Results approximated those obtained in the case of Example I.

Example VIII In this example, the procedure of Example I was again duplicated, but substituting an acrylate methacrylate copolymer resin for the butadiene styrene polymer of the first example. The procedure and results followed approximately along the lines of the earlier examples. The acrylate methacrylate copolymer utilized in this example was procured under the trademark Acryloid A-101 from the Rohm & Haas Company.

Example IX In this example, the butadiene styrene copolymer resin of Example I was replaced by an aldehyde alkyd resin in which the aldehyde portion dominated the polymeric structure. This particular resin was purchased from the General Electric Company under the designation Glyptal #1202.

Example X 2.0 grams Furnace Black Pigment, 7.0 grams Epon 1001, 14 cc. toluene and 1 cc. butanol were combined or milled to form a uniform paste using the same procedure given in Example I. One gram of the resulting paste was diluted with 25 cc. of toluene to form a stock solution. The latter was dispersed with Freon 113 in the usual manner to form a toner bath. When this toner composition was employed for developing images in accordance with the procedure of Example I, a lustrous black image was produced which was further characterized by excellent adhesion to the substrate. The superior toned images obtained in this example can be attributable to the excellent polarity control of this epoxy resin.

8 Example XI 2.0 grams of a 91% vinyl chloride, 9% vinyl acetate copolymer resin (purchased from the Bakelite Division of Union Carbide Corporation under the trademark VAGH) was dissolved in ml. of a 50-50 mixture of toluene and methylethyl ketone to form a fixer solution. 15 grams of a green lithographic offset ink (Colitho Bright Green CO 28) was dispersed by conventional means in 100 grams of toluene to form a green toner stock solution. 10 ml. of the fixer solution was combined with 10 ml. of the stock solution and the resulting mixture diluted with 280 ml. of Du Pont Freon 113 to form a negative rapid fixing toner bath. Negative charge image patterns, formed on an electrophotographic member comprising a humidified paper base having coated thereon particles of zinc oxide suspended in electrically insulating medium, were fully developed to a white image on a green background upon immersion for approximately 5 seconds in the toner bath containing an auxiliary negative control electrode as an aid for repulsion development. After removal from the developing bath, the image and background were resistant to smearing or smudging as soon as the solvent had evaporated. The efliciency of the toner formulation described above can be tested with the apparatus consisting of coplanar insulated charged metallic plaques described in Example I. Examination of the paper test sheet after evaporation of the Freon 113 carrier fluid from a sample of liquid toner showed a pattern of green spots corresponding in size, shape and location to the positively charged plaques of the test instrument. In addition, the regions on the paper directly over negatively charged plaques were entirely free of green toner even though the surrounding paper areas did absorb some green coloration.

Example XII The procedure of Example XI was followed except that the green lithographic ink was replaced by Colitho Black 1A. Results paralleled those of Example XI.

Example XIII The prepared ink of Example XII was replaced by a pigment having a negative zeta potential, to wit: General Dyestuffs Pigment Heliogen Green Toner 663001. 1.0 grams of this pigment was dispersed in 300 cc. of Du Pont Freon 113 containing 10 ml. of the fixed solution of Example X. This formulation, when tested according to the procedures of Example X, gave con1- parable results.

Example XIV In this example, the lithographic ink was again the Colitho Bright Green C028 utilized in Example XI as the pigment. However, the fixer stock solution consisted of 2.0 grams of a styrene-acrylonitrile modified alkyd resin (purchased from the American Cyanamid Company as Cycopol 320-5) dissolved in 100 cc. toluene. As in the case of Example XI, this combination of resin and lithographic ink resulted in well fixed negative prints.

Example XV The vinyl chloride-vinyl acetate copolymer resin of Example XI was replaced by polyvinyl butyral (vinyl ether) resin (manufactured by Shawinigan Resins Corp. as Butvar). When tested with the green lithographic ink of Example XI, similar results were obtained.

Example XVI In this example, an alkyd resin in which ester linkages dominate the polymeric structure (Rhodene L6/ 100, manufactured by Polymer Corp. of Australia) replaced the vinyl copolymer resin of Example XI with approximately similar results.

We claim:

1. A liquid developer for developing negatively charged electrostatic images into visible negative images consist- 9 ing essentially of a dispersion of a hydrophobic pigmented lithographic ink in a carrier liquid having a specific resistivity of at least about 10 ohm centimeters and having dissolved therein in addition a polymeric fixing agent selected from the class consisting of a styrene and butadiene copolymer having the recurring unit:

an acrylate and methacryl-ate copolymer having the recurwhere R and R are selected from the group consisting of hydrogen and lower alkyl; and an epoxy resin having the following recurring unit:

r i i i CH3 .J said dispersion being further characterized in that the lithographic ink and the fixing agent each has a maximum zeta potential of the same positive electrical polarity, the ratio by weight of pigment in the lithographic ink to resinous fixing agent varying between 10:1 and 1:10.

2. A liquid developer for developing electrostatic images into positive visual images consisting essentially of a dispersion of hydrophobic pigmented lithographic ink in a carrier liquid having a specific resistivity of at least about 10 ohm centimeters and having dissolved therein in addition a polymeric fixing agent selected from the class consisting of a styrene-acrylonitrile modified alkyd resin; a vinyl ether resin having the recurring unit:

wherein R is a member of the class consisting of lower alkyl, phenyl and toluyl groups, and a poly(vinyl alcohol-paranitrophenyl ether)resin having the recurring unit:

said dispersion being further characterized in that the ink and fixing agent each has a maximum zeta potential 10 of the same negative electrical polarity, the ratio by weight of pigment in the lithographic ink to resinous fixing agent varying between 10:1 and 1:10.

3. A developer according to claim 1 wherein the carrier liquid is essentially a liquid paraflin hydrocarbon.

4. A developer according to claim 1 wherein the carrier liquid is essentially 1,2,2-trichloro-1,1,2-trifiuoroethane.

5. A developer as defined in claim 1 wherein the fixing agent used in developing the negative images is an epoxy resin having the following recurring unit:

6. A developer according to claim 2 wherein the carrier liquid is essentially 1,2,2-trichloro-1,1,Z-trifiuoroethane.

References Cited by the Examiner UNITED STATES PATENTS 2,851,373 9/1958 Tregay et al 117--17.5 2,907,674 10/ 1959 Metcalfe et al. 11737 3,010,842 11/1961 Ricker 11737 3,032,432 5/1962 Metcalfe et a1. 117 37 3,041,168 6/1962 Wielicki 117-17.5 $058,914 10/1962 Metcalfc et a1. 252-62.1 3,078,231 2/1963 Metcalfe et al. 26262.1 3,081,263 3/1963 Metcalfe et al. 252-621 40 JULIUS GREENWALD, Primary Examiner.

WILLIAM D. MARTIN, ALBERT T. MEYERS,

Examiners.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,244,633 April 5, 1966 Wilbur Yellin et al.

It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 9, lines 7 to 11, the formula should appear as shown below instead of as in the patent:

CH2 -cH=cH-cH -cH-cH Signed and sealed this 19th day of November 1968.

(SEAL) Attest:

EDWARD J BRENNER Edward M. Fletcher, Jr.

Commissioner of Patents Attesting Officer 

1. A LIQUID DEVELOPER FOR DEVELOPING NEGATIVELY CHARGED ELECTROSTATIC IMAGES INTO VISBLE NEGATIVE IMAGES CONSISTING ESSENTIALLY OF A DISPERSION OF A HYDROPHOBIC PIGMENTED LITHOGRAPHIC INK IN A CARRIER LIQUID HAVING A SPECIFIC RESISTIVITY OF AT LEAST ABOUT 10**10 OHM CENTIMETERS AND HAVING DISSOLVED THEREIN IN ADDITION A POLYMERIC FIXING AGENT SELECTED FROM THE CLASS CONSISTING OF A STYRENE AND BUTADIENE COPOLYMER HAVING THE RECURRING UNIT: 